The present invention relates to a golf ball mold suitable for molding solid golf balls composed of a core encased by one or more cover layer, and thread-wound golf balls. The invention relates also to golf balls molded using such a mold, and to a method of manufacturing golf balls using such a mold.
Molds for molding golf balls are generally composed of a plurality of parts which removably mate to each other; a golf ball is manufactured by feeding a golf ball molding material to a cavity that forms at the interior of the mold when these mold parts are mated. From the standpoint of ease of mold fabrication and ball moldability, etc., the parting plane of each mold part is often rectilinear in shape without concavities and convexities. Parting planes having such a rectilinear shape are often coincident with the equator of the golf ball. Thus, in golf balls molded with such a mold, dimples are not formed on the equator which corresponds to the parting plane; instead, a somewhat wide great circle forms at the equator.
However, in a golf ball having at the equator a wide great circle across which there lie no dimples, it is difficult to achieve a uniform arrangement of dimples on the spherical surface of the ball. This leads to a lack of uniformity in the aerodynamic symmetry of the ball, giving rise to a variability in the flight performance due to differences on where the ball is hit.
For this reason, efforts have been made to form dimples which lie across the equator and thereby eliminate a wide great circle on the equator. For example, JP-A 10-127826 discloses a golf ball mold 10 having a construction wherein, as shown in FIG. 8, an upper mold half 10a and a lower mold half 10b removably mate to form at the interior a hollow spherical cavity c having an inner wall with numerous dimple-forming protrusions 40 thereon. In addition, the parting planes 30 on the upper and lower mold halves are formed in concavo-convex shapes, and dimple-forming protrusions 40 are situated so as to lie across the parting line PL at the concavo-convexly shaped areas. In this mold 10, to form dimples which lie across the parting line PL, cylindrical pins (convex features) 30a having dimple-shaped ends are provided on the parting plane 30 of the lower mold half 10b and circular holes (concave features) 30b corresponding to the cylindrical pins (convex features) 30a are formed on the parting plane 30 of the upper mold half 10a so that these fit together when the upper and lower mold halves are mated. Also shown in the diagram is a resin injection port 20.
In addition to the foregoing, numerous disclosures have been made wherein, to have dimples lie across the golf ball equator, the parting plane of the mold is given a shape that is concavo-convex rather than rectilinear, with all or part of a dimple-forming protrusion being disposed on the convex portions thereof (e.g., JP-A 06-143349, JP-A 08-173576, JP-A 11-070186, JP-A 11-137727, JP-A 2001-170217, JP-A 2001-187172, JP-A 2002-159598, JP-A 2004-089549, JP-A 2006-212057, JP-A 2007-136182, JP-A 2007-159715 and JP-A 2007-268265).
At the same time, when one takes into account the aerodynamic properties of golf balls, providing dimples on a greater portion of the golf ball surface is desirable, such desirable effects being known to increase as the total surface area of the dimples as a proportion of the golf ball surface area (surface area coverage) approaches 100%. Hence, to improve the symmetry of the aerodynamic properties in this way and achieve an even further increase in performance, bringing the dimple surface coverage close to 100% is important.
Generally, in this type of mold, resin injection ports (also referred to below simply as “injection ports”) for introducing resin into the cavity are provided along the parting plane. However, circular injection ports having an opening with a specific surface area to keep imbalances in the injection pressure and resin flow rate from arising are generally formed at sizes and positions which do not overlap with the dimple-forming protrusions (on the molded ball surface, at positions which correspond to lands where dimples do not form). For example, JP-A 2000-42143 mentions providing circular injection ports having a diameter of about 0.5 to about 1.0 mm. However, the intervals between the dimples become smaller as the dimples are arranged more densely in order to bring the dimple surface coverage closer to 100%; as a result, the surface area available for providing injection ports decreases. To address this, it is common to reduce the diameter of the injection ports. Yet, imbalances in the resin injection pressure or flow rate often arise. Hence, when a cover is formed over a core or a sphere composed of a core encased by another layer such as an intermediate layer, the core may end up deformed or the position of the core within the mold may end up off-center, leading to various problems, such as molding defects, scorching or eccentricity. Golf balls with these problems have an inferior quality, such as durability, symmetry and appearance. Also, as shown in FIG. 9, by adjusting the sizes of the dimple-forming protrusions 40 and making the interval between those dimple-forming protrusions 40 which adjoin the injection port 20 larger, it is possible to secure an area adequate for the surface area of the opening in the injection port 20, although further increase in the uniformity of the aerodynamic properties or the surface coverage is difficult.
Thus, various innovations have hitherto been made to increase the dimple surface coverage and thereby enhance the aerodynamic properties. However, a fundamental solution to the conflicting problems of mold design and dimple design has yet to be achieved. In order to further enhance the aerodynamic properties of the golf ball and also improve the degree of freedom in mold design, an approach that resolves the above problems has been sought.